The use of a double clock extractor in a passive type transponder circuit is known in particular from patent EP 1 301 898. The passive transponder circuit comprises an antenna to pick up a sinusoidal electromagnetic data or control signal coming from a reader close by. The antenna is connected to the double clock extractor and to a rectifier that produces a stable rectified voltage for supply to sections of the transponder circuit on the basis of the picked up electromagnetic signal. The transponder circuit also comprises a demodulator, a logic control circuit and a modulator. The demodulator is connected between the double clock extractor and the logic circuit, whereas the modulator is controlled by the logic circuit for transmission of a data signal via the antenna.
The first low clock extractor of the transponder circuit of patent EP 1 301 898 is formed by means of two single inverters, of which the switching threshold voltage is close to 1 V. A clock signal fed to the demodulator is produced by this first clock extractor only if the amplitude of the signal picked up by the antenna is higher than 1 V, otherwise the clock signal is interrupted. The second high clock extractor is formed by means of a PMOS input transistor, a polarised inverter controlled by the input transistor, a rectifier and two current sources. A clock signal fed to the demodulator is produced by this second clock extractor only if the amplitude of the signal picked up by the antenna is higher than a high switching threshold higher than the threshold of the first clock extractor.
When the reader interrupts the transmission of the sinusoidal signal during a specific period, the amplitude of the signal received by the antenna of the transponder circuit decreases slowly. The clock signal of the second high clock extractor is interrupted before the clock signal of the first low clock extractor. Because of this, the double clock extractor with two levels of different thresholds only assures a pause of very short duration before a new transmission of the sinusoidal signal of the reader. However, this transponder circuit with double clock extractor is not provided to detect an interruption in the magnetic field of the signal produced by the reader before a demodulation of data or commands subsequent to the interruption in the field. Moreover, a selection of the clock signal of one clock extractor or the other is not provided for the demodulator in the case where the modulator is in operation, since the modulator must not be in operation during receipt of the electromagnetic signal.
For various low-power applications, a passive transponder circuit can comprise a single sensitive clock extractor, which requires a sensitivity close to 100 mV. This sensitive clock extractor can be used even if the amplitude of the signal picked up by the antenna coil of the transponder circuit is below 150 mV at maximum, when a switch of the modulator is in a switched-on or conductive status ON.
In order to start a demodulation of data received in the alternating magnetic field of the signal picked up by the antenna coil, an interruption in the field of the picked up signal generated by an adjacent reader must firstly be detected in general. However, a transponder circuit fitted with a sensitive clock extractor is not generally able to detect this field interruption when the switch of the modulator is in a switched-off status OFF. In this switched-off status OFF, with a first interruption in the alternating magnetic field of the reader, a float in voltage on the coil is observed without this voltage reaching earth potential. The reason for this is that the resistor or resistors of the modulator are not arranged in parallel with the antenna of the resonant circuit as is the case when the switch is in a switched-on status ON. Therefore, there is a rearward voltage leak towards the antenna coil, since the rectifier does not operate below a switching threshold corresponding to that of an inverter, e.g. in the order of 0.6 V.
If the voltage at the coil is stabilised, for example, to 300 mV, everything will depend on the threshold of the clock extractor with respect to the detection of an interruption in the field of the picked up signal. As a result, since the clock extractor is sensitive from 100 mV, detection of the interruption in the magnetic field of the picked up signal cannot occur in principle, if the voltage at the coil is stabilised to 300 mV.